1. Field of the art
This invention relates generally to the production of acrylic acid, particularly to that by two-stage, vapor-phase, catalytic oxidation of propylene. More specifically, the invention relates to a process for producing acrylic acid by oxidizing propylene in a high concentration.
As a process for producing acrylic acid, the process of catalytically oxidizing propylene in two stages in the gas phase by using air is known and has already been reduced to industrial practice. In the first stage of this process, propylene is mixed and supplied with air and steam or an inactive gas such as nitrogen thereby to convert the propylene principally into acrolein and into acrylic acid in a by-procuct quantity. This first-stage outlet gas thus formed is fed as it is without separation of the products formed to the reaction vessel of the second stage.
Modification of this two-step method is also known where oxygen required for causing reaction in the second stage or steam is added anew to the above mentioned outlet gas.
In the second stage, the acrolein is converted principally into acrylic acid. The acrylic acid thus formed is generally cooled and, is separated as an aqueous solution thereof, and recovered from the gas flow, being passed through processes such as extractive distillation in the succeeding purification process step, thereby being isolated. The alternative method of precooling the second-stage outlet gas flow and then absorbing the acrylic acid with a suitable solvent thereby to separate the acrylic acid has also been proposed.
In this catalytic oxidation, the steam, which is one component of the feed gas, is necessary for the catalysis in the second stage wherein the acrolein is oxidized into acrylic acid. In addition, the steam serves also as a diluent for reducing the danger of explosion due to the mixing of the propylene or acrolein with oxygen to form an explosive gas mixture. However, if steam is used as a diluent in a great quantity, the aqueous solution of acrylic acid obtained in the recovery of acrylic acid by the ordinary cooling condensation method will be dilute, which will give rise to disadvantages such as an increase in the cost of separating the acrylic acid from the solution or an increase in the recovery loss of acrylic acid. Furthermore since the steam, itself entails a high cost, the use thereof in a great quantity, in any case, is uneconomical.
2. Prior art
For this reason, a method wherein, as a diluent for preventing the formation of an explosive composition within the range of inflammable mixture, a portion of the waste gas remaining after recovery and separation of acrylic acid and water, etc., from the outlet gas of the second-stage reactor by cooling, recovery with a solvent, or some other method, is recycled and substituted for steam has been proposed. The waste gas thus recycled comprises principally nitrogen, carbon dioxide, carbon monoxide, etc., but, depending on the reaction conditions, contains also unreacted propylene, acrolein, oxygen, and other gases. For example, Japanese Pat. Publn. No. 30688/ 1978, Pat. Laid Open Publn. No. 108917/1977, and Pat. Laid Open Publn. No. 15314/1978 disclose methods wherein the reaction waste gas is recycled to the first-stage inlet. Japanese Pat. Laid Open Publn. No. 36415/1976 discloses a method wherein the waste gas is divided and recycled to the first stage and the second stage.
In these processes, on the one hand, since the performances of their oxidation catalysts greatly influence their economies, a large number of proposals concerning catalysts for each stage have been made.
For example, catalysts for the oxidation of propylene into acrolein in the first stage are disclosed in Japanese Pat. Publn. Nos. 17711/1972, 27490/1972, 41329/1972, 42241/1972, 42813/1972, 1645/1973, 4763/1973, 4764/1973, 4765/1973, and others. Catalysts for the oxidation of acrolein into acrylic acid in the second stage are disclosed in Japanese Pat. Publn. Nos. 12129/1969, 19296/ 1973, 169/1974, 11371/1974, 10432/1977, and 31326/1977, and Japanese Pat. Laid-Open Publn. Nos. 2011/1971, 8360/ 1972, 43922/1974, 6117/1974, 124016/1974, 133317/1974, 25520/1975, 93918/1975, 23589/1977, 29483/1977, 29484/ 1977, and others.
Most of these catalysts are indicated as affording one-pass yields of the desired product of the order of 90 percent or higher, and it may be considered that, for the time being, sufficiently high yields of the desired product for economical carrying out of the processes of each stage are being obtained.
However, with respect to the object of economically producing acrylic acid, the prior and present technology cannot be said to be fully satisfactory. One important factor affecting this state of the art relates to the compositions of the feed materials. More specifically, first, an equimolar quantity of oxygen should be sufficient, theoretically, for oxidizing propylene to form acrolein in the first stage, and, if there are no other restrictions, the use of a volume of air which is 4.76 times relative to that of propylene, for example, will be sufficient. The propylene concentration in the gas mixture is thus 17.4 percent. In actual practice, however, a range of the propylene concentration of 4 to 7 percent is used in most cases. Accordingly, it can be said surplus gas which is excessive by the difference is being fed into the reactor.
As the materials to be fed into the second stage, oxygen required for the conversion of the acrolein formed in the first stage into acrylic acid (the theoretical oxygen quantity being 1/2 mol of the acrolein) and steam in a quantity of the order of the same mol or more in view of the catalysis are necessary. Therefore, in the case where the feed gas to the first stage has a composition near the lower limits, replenishing of the oxygen and steam is necessary in an intermediate stage. However, various difficulties must be overcome in order to bring an industrial process close to such an ideal, and corresponding technical measures are necessary. One such measure is to cause the selectivity of the catalyst to approach 100 percent of the desired product. A second is a safety measure in the process.
The process of feeding starting gas of a high propylene concentration into the first stage and, at the outlet thereof, replenishing oxygen and steam and feeding the resulting mixture into the second stage thereby to produce acrylic acid is known through Japanese Pat. Laid Open Publn. No. 25521/1975. According to this publication, the desired product was obtained with a high spacetime yield by using a specific feed composition and a specific catalyst. However, according to our knowledge, there appears to be an unreasonable feature in this process in the removal of heat, and, moreover, there is no safety measure whatsoever, whereby it is not possible to apply this process to an industrial apparatus.
The measures which must be taken with regard to safety in the process are the avoidance of forming an explosive composition of propylene, acrolein, etc., and the prevention or suppression of a runaway or uncontrollable combustion reaction due to spontaneous oxidation of the acrolein at the outlet of the first stage. As a measure for avoiding the formation of an explosive composition, the admixing of an inactive gas such as steam, nitrogen, and carbon dioxide gas with the explosive gases thereby to form a gas outside of the explosion range (or range of inflammability) is generally resorted to. Since this explosion range varies with factors such as temperature, pressure, and the diluting gas, the selection of the conditions is important.
With respect to the combustion of acrolein at the first-stage outlet, the following countermeasures, for example, have been proposed. Japanese Pat. Laid Open Publn. No. 132007/1974 discloses a method wherein the reaction product gases are introduced from the outlet of the reaction zone directly into a directly adjoining heat-exchange zone and are cooled to a temperature of 200.degree. to 300.degree. C. by using water. Japanese Pat. Laid Open Publn. No. 36415/1976 discloses a method wherein air and waste gas are added to and mixed with a reaction gas mixture immediately after it has been discharged from the catalyst zone at a point to the rear of the first stage under rapid cooling to a mixing temperature of 150.degree. to 320.degree. C. Japanese Pat. Laid Open Publn. No. 15314/1978 discloses a method of restricting the oxygen concentration and adjusting it to a range of 1.2 to 1.6 times in terms of mols the propylene concentration.
In the method of Japanese Pat. Laid Open Publn. No. 132007/1974, however, there arises a problem in that the acrolein undergoes combustion before ample cooling can take place within the tubes of the heat-exchange zone. In the method of Japanese Pat. Laid Open Publn. No. 36415/ 1976, air and waste gas are added to the reaction gas mixture, but before these gases are thoroughly mixed and cooled, combustion of the acrolein similarly occurs. In the method of Japanese Pat. Laid Open Publn. No. 15314/ 1978, residual oxygen always exists because oxygen in excess of the oxygen required for the oxidation of the propylene is used, and under this condition it is virtually impossible to suppress the combustion of the acrolein. The methods of the above cited references are accompanied by these and other problems.
Thus, while these proposed methods have some meritorious effectiveness, it is difficult to say that these methods are fully satisfactory, and their practical application to industrial apparatus have still been inadequate.